Heat-shrink conveyor systems are utilized in the packaging industry to replace cardboard cartons with, in many cases, a cardboard tray and a thin film of plastic material which when heated forms around the containers to physically hold the product in place. Examples of such products include water, soda, and other flavored drink bottles or can products from soda to soup or vegetables. Electric operated heaters have been utilized to provide the elevated air temperature required to activate the “shrink” feature of the thin-film plastic material. The electric heaters normally have difficulty in sustaining a constant and uniformly disseminated elevated heat to achieve the shrink wrapping operations, and secondly, does not accommodate uniformity of circulation of the heat to assure that all aspects of the shrink wrap film is constantly exposed to the same temperature, to assure consistent and uniform shrinkage of the film during its packaging of the desired goods.
The conveyor system includes an enclosure above a moving conveyor which forms a tunnel to contain the heated air around the product passing through the chamber. The heated air passes from the chamber back across the electric heater elements and then into the re-circulating fan where it discharges back into the chamber in a re-circulating manner. Drawing air across an electrical heating bank that spans the width of the tunnel attempts to create an even temperature distribution in the chamber where the plastic material shrinkage is to take place. It is very important that the temperature in the chamber be uniform so that the plastic material shrinks evenly against the packaged product.
The operating temperature needed to accomplish the proper shrink effect in a tunnel requires that temperatures to be in excess of 350° F., however, the speed of the product moving through the tunnel may require the chamber temperature to reach temperatures approaching 450° F. The specified temperature must also be held at the set-point within very close tolerances generally within plus or minus 1%. This is important should the volume of product flowing through the tunnel change abruptly. Thus the need for a fast responding modulation controls system along with the requirement for a high turn down ratio of the heating system.
The above system temperature requirements make it difficult to address with indirect gas-fired burner/heat exchanger systems. Space for the heater section is generally limited which creates a problem for a heat exchanger that has to be de-rated at these operating temperatures to keep from exceeding the temperature limit of the material and to maintain the thermal efficiency near 80%. The lower efficiency of the heat exchanger makes it more difficult to compete against the cost of electricity in most parts of the country.
Attempting to apply a direct fired-gas burner system to such an application would have an advantage over an indirect approach because it has a 100% thermal efficiency by its very nature, however, it has been problematic because the heat source is much closer to a point source than distributed as a plane section source as is accomplished by the electric heater bank. The airflow pattern in the tunnel acts to keep the heat flow from a point source burner from reaching the far side of the tunnel, so one cannot mount a burner on one side of the chamber and expect that the temperatures will eventually equalize. Furthermore, the confines of the conveyor cabinet limits the amount of air mixing devices that can be added to match the uniform temperature profile produced from the electric heating system. A solution to this condition is therefore the basis for this patent application.
The operating cost difference between electrical power cost and the cost of natural gas operating at a thermal efficiency of 100% more than justifies a solution to overcome the obstacles that had thus far blocked the entry of a direct gas-fired system from breaking through as a viable alternative to the electric heater in this market.
It becomes obvious that other specialized-heating applications that have similar issues as are encountered with the shrink-wrap conveyer systems as described above would benefit from the solutions offered by this invention. This is a common problem when the heat source tends to act as a point source rather than a distributed output such as a planer source like a heating element bank when the airflow is perpendicular to the heat source. A similar problem exists when a point heat source is small in comparison to the plane of the airflow pattern when the airflow is in the same directions as the heat output. The velocity of the system fan tends to keep the high temperature air from mixing evenly.